1. Field of the Invention
The present invention relates to liquid distributors for vapor-liquid contact columns and, more particularly, to a liquid distributor assembly for packed columns wherein counter-current, vapor-liquid flow is employed.
2. The State of the Prior Art
It is well known in the prior art to utilize various types of exchange columns or towers, in which a gas and a liquid are brought into contact with one another, preferably in counter-current flow for purposes of mass and/or heat transfer, close fractionation and/or separation of feedstock constituents, and other unit operations. The overall process generally involves mass transfer, heat transfer, fluid vaporization and/or fluid condensation. To conduct these operations efficiently, intimate contact between the gas and the liquid must be achieved and yet the pressure drop within and the physical dimensions of the contact zone desirably is minimized. For these reasons, counter-current flow of vapor and liquid within the exchange column has become the preferred method for bringing the vapor and the liquid into the desired intimate contact. Moreover, the preferred means for achieving this desired contact is a so-called packed bed positioned within the column. Such beds are well known. Some are packed with a multiplicity of packing elements designed to maximize the vapor/liquid interface. Others may be in the form of a structured packing.
Liquid is distributed on top of these packed beds in the most feasible manner, while vapor is distributed beneath the beds in the lower regions of the column. In this manner, liquid trickling downwardly through the packing bed is spread out over the surfaces of the packing elements so as to maximize the surface area of the liquid that is exposed to vapor ascending through the bed.
It is well established that the configuration of the packing bed determines the pressure drop, efficiency of the vapor-liquid interface and the concomitant mass and energy transfer occurring in the process column. The means for effective and even distribution of the vapor and the liquid on opposite sides of the packing bed as well as maintenance of that distribution as the fluids move through the packing are critical to the efficient operation of the column. Only with efficient initial vapor and liquid distribution and the maintenance of such distribution throughout the packing, will homogenous mixing zones be created so as to maximize the efficiency of the overall contact operation. Efficiency is readily convertible to cost of operation and production quality. For this reason, a myriad of prior art packing designs have been prevalent in conventional exchange columns.
The efficiency of the packing, however, is limited to a large extent by the efficiency of the methodology used to distribute the vapor and liquid across the bed. For example, failure of either vapor or liquid to be evenly distributed over the entire cross-section of the bed effectively eliminates the utility of the part of the packing where there is poor or no distribution. This in turn adversely directly affects the overall cost effectiveness and efficiency of the contact operation. The packing bed depths are critical in establishing production criteria and operational costs, and failure to evenly distribute both the vapor and the liquid across the packing and/or to maintain homogeneity within the packing can lead to serious consequences, particularly in the petroleum refining industry.
Aside from the packing beds, the liquid distributors are the most important components of the column. Failure in performance of a packed column sometimes stems from liquid distribution problems such as clogging or uneven distribution. Hence, the selection of a proper liquid distributor is critical for uninterrupted continuous operation. Operational considerations thus include the functional aspects of the distributor, such as how level the distributor troughs are maintained, how well the flow is equalized therethrough, and the means through which the liquid is distributed from the troughs to the packing beds positioned beneath the distributor. Also considered is the effect which the ascending vapor has on the liquid being distributed. When vapor flow areas are restricted, flow velocity can increase to the point of interrupting the descending flow pattern. The liquid is, in essence, xe2x80x9cblownxe2x80x9d around, and this condition can lead to uneven distribution and inefficiency in the operation of the column.
Conventional liquid distributors generally include the multi-orifice spray head variety adapted for dispersing liquid in the form of a spray atop a packing bed. In the utilization of dump packing wherein a plurality of randomly oriented packing elements are disposed, within the exchange column, such liquid distribution technique is sometimes effective. This is particularly true when high efficiency parameters are not of critical significance. However, in the event of high efficiency packing such as that which is described in U.S. Pat. No. 4,604,247, means for homogenous liquid and gas distribution is of extreme importance.
The cost of high efficiency packing of the type described in the ""247 patent mentioned above commands attention to proper vapor-liquid distribution. Even small regions of non-homogenous interaction between the vapor and liquid is an expensive and wasteful loss not consistent with the utilization of high efficiency packing, where space and homogeneity in vapor-liquid interface is both expected and necessary for proper operation. High efficiency packings of the state of the art varieties shown in the ""247 patent require counter-current vapor-liquid flow through the channels defined by the opposed corrugations of sheets. If the initial liquid or gas distribution fails to enter a particular corrugation pattern, then precious surface area is lost in the packing until the liquid and vapor are urged to migrate into and interact through the unfilled regions of the packing. Only by utilizing proper vapor and liquid distribution means may effective and efficient utilization of high efficiency packing as well as conventional dumped packing be assured.
The development of systems for adequate liquid distribution in process towers has been limited as set forth above. In the main, it is known to discharge and distribute liquids with spray orifices, pipes, perforated plates, apertured troughs and nozzles. Gas is concomitantly discharged in an ascending turbulent configuration to provide adequate vapor distribution. Although many prior art systems are generally effective in distributing some vapor and some liquid to most portions of the packing bed, uniform distribution across the entire bed is usually not obtained without more sophisticated distribution apparatus. For example, unless gas is injected into a myriad of contiguous areas beneath the packing bed with equal pressure in each area, the mass flow of vapor upwardly through the packing bed cannot be uniform. Random vapor discharge simply distributes unequal amounts of vapor across the lower regions of the packing bed but does not in any way assure equality in the distribution. Likewise the simple spray of liquid atop the packing bed, though intended to be effective in wetting all surface areas, often results in high concentrations of liquid flow in certain packing bed areas and less flow in others. This, of course, depends on the spray device. Orifice distributors are generally more susceptible to plugging than other types of distributors, and plugging is generally non-uniform to uneven irrigation within the tower. Surface irregularities in a distributor pan occurring during manufacture likewise increase flow resistance of some perforations or induce liquid flow along the bottom of the pan which is a distinct disadvantage. Any flow irregularity which focuses the flow in one area while reducing flow in other areas is deleterious.
When pipe distributors consisting of headers equipped with tributary pipes or laterals that have holes or nozzles to spray liquid are used, the liquid is often distributed too finely. Tiny droplets of the liquid then get carried out of the tower by counter-current gas flow. This may sometimes prevent the liquid from even coming into contact with the packing bed. Since liquid-vapor contact is the purpose of the packing, such a result totally frustrates the intent of the liquid distributor. As much as 5% of the liquid flowing through a nozzle can be converted to mist at a pressure drop of 20 psi. It has also been noted that nozzle equipped pipe distributors can produce overlapping spray patterns which result in increased flow in certain areas with reduced flow in other areas. Moreover, spray headers also release liquid at speeds that can cause it to pass vertically through the packing before it has a chance to spread out horizontally depending on the particular packing type.
These issues are important and often the most critical issues determining the number of liquid distribution points necessary for various tower diameters, packing heights, materials and systems. Moreover, with randomly packed beds it is critical that the packing height not be too great wherein the weight of the packing will cause it to crush itself. However, liquid redistributors between packing sections are expensive and take up heights that could otherwise be used for mass transfer. An important consideration is often the type of packing being used. For example, structured packing can tolerate very little maldistribution while dump packing on the other hand can sustain larger variations in liquid distribution.
Unfortunately, the manifestation of uneven liquid distribution generally occurs in the vicinity of the most even, or uniform, vapor distribution. The opposite is also true. This is because vapor has had a chance to more evenly distribute through the packing bed prior to engaging the liquid distribution flow. It would be an advantage, therefore, to provide even distribution for both liquid and vapor prior to entry of the vapor and liquid into the packing bed. Ideally, there should be both a uniform spread of liquid and vapor and uniform volumetric distribution in the bed.
A highly efficient liquid distributor which addresses many of the concerns discussed above is described in U.S. Pat. No. 4,909,967, the entirety of the disclosure of which is hereby specifically incorporated herein by reference thereto. The ""967 patent describes an improved system of vapor-liquid distribution through a tube-trough distributor wherein each trough is constructed with a plurality of tubes that depend below the bottom region thereof so as to prevent disturbing the rise of vapor flow at the point of liquid discharge. The present invention provides even better improved results in many process applications than the distributor of the ""967 patent.
The present invention addresses the problems and shortcomings of the prior art by providing a novel flow distributor for use in process columns. In particular, the invention provides a flow distributor that is of simple construction and which operates efficiently with a minimum of pressure drop so as to maximize of liquid-vapor contact in the packed bed. In accordance with its broadest aspects, the invention provides an improved flow distributor for liquid descending in a process column. The distributor includes an elongated liquid trough having first and second laterally spaced, longitudinally extending sidewalls and a longitudinally extending floor which extends between and interconnects said sidewalls. A liquid drain hole is provided in the first sidewall. The distributor also includes an elongated liquid distributor drain pipe having first and second ends and a central internal liquid conducting passageway extending therethrough from one of its ends to the other. The pipe has a first segment adjacent the first end and a second segment spaced longitudinally of the pipe from the first end and such segments are angularly disposed relative to one another. The pipe is connected to the sidewall with the passageway at the first end of the pipe in essential alignment with the drain hole, with the first segment extending laterally and outwardly away from an outer surface of the first sidewall and with the second segment extending downwardly in outwardly spaced relationship relative to such outer surface.
In accordance with one of the preferred embodiments of the invention, the distributor further may include a connector assembly that removably attaches the first end of said pipe to the first sidewall. The connector assembly may include a receptacle mounted on the first sidewall and an attachment element on the pipe at its first end. The receptacle may preferably be configured for receiving the attachment element therein and holding the pipe in its desired orientation with the passageway at the first end of the pipe in essential alignment with a corresponding drain hole with the first segment of the pipe extending laterally and outwardly away from an outer surface of the first sidewall, and with the second segment of the pipe extending downwardly in outwardly spaced relationship relative to such outer surface.
Desirably, the first sidewall may have a plurality of holes therethrough, and such holes may be spaced apart longitudinally of the distributor. In this case, the distributor may include a respective drain pipe for each hole. The distributor may also include a respective connector assembly for each drain pipe.
Ideally, each of the sidewalls may have a plurality of holes therethrough, and such holes may preferably be spaced apart longitudinally of the trough. In this case, the distributor preferably has a respective elongated liquid distributor drain pipe as described above for each of the holes. As before, the pipes may be attached to a respective sidewall with the passageway at the first end of the pipe in essential alignment with a respective hole, with the first segments extending laterally and outwardly away from an outer surface of the respective sidewall to which each pipe is attached, and with said second segments extending downwardly in outwardly spaced relationship relative to the outer surface of the respective sidewall to which the pipe is attached.
Each of the pipes may have a delivery end that is disposed at a lower elevation than the floor of the trough. The arrangement may be such that the delivery end of the pipe is disposed directly beneath the trough. Alternatively, the arrangement may be such that the delivery end of the pipe is disposed in laterally, outwardly spaced relationship relative to the trough. Ideally, the delivery ends of one group of pipes may be disposed directly beneath the trough and the delivery ends of another group of pipes may be disposed in laterally, outwardly spaced relationship relative to said trough. In its most preferred form, the distributor of the invention may include a plurality of the elongated liquid troughs and such troughs may be arranged in parallel, laterally spaced relationship.
In a particularly preferred form of the invention, the receptacle of the connector assembly may comprise a generally U-shaped channel element having a generally flat base member and a pair of generally parallel, laterally spaced flanges located on opposite side edges of the base member. The base member desirably has a hole therethrough that is in alignment with a corresponding hole in a sidewall when the receptacle is mounted in an operational position. Ideally the flanges may extend vertically when the receptacle is mounted in an operational position.
The attachment element may be in the form of a generally planar, generally rectangular plate having a lateral dimension that coincides essentially with the distance between the flanges of the channel element. Desirably, the plate and the channel element are configured such that the plate is held against the base member by the flanges. In its most efficient form, the receptacle may include a stop in the base member which contacts a lower edge of said plate to hold the plate in a proper operating position and a spring clip to hold the plate tightly against the base member.
The invention also provides a process column that includes a packed bed and a liquid flow distributor as set forth above for distributing liquid across a top face of said bed. The invention also provides a method for operating a process column that involves the steps of providing a packed bed in the column and distributing a descending flow of liquid across a top face of said bed utilizing a distributor as set forth above.